The general aim of this grant proposal is to elucidate the role of corticothalamic connections from anterior parietal cortex (APC) to the ventrobasal (VB) thalamus in the processing of somatic sensory information I adult non-human primates. Most studies concerning the processing of sensory input have focused on the ascending flow of information from thalamus to cortex, despite the fact that descending or "feedback" projections are in many cases more numerous than "feedforward" projections and provide a substantial substrate for influencing the extraction and detection of pertinent sensory information. Most of what is known concerning the functional significance of corticothalamic projections has come from studies conducted in the visual system, which have indicated that this pathway is capable of altering the receptive field properties of thalamic sensory relay neurons. While a few studies have attempted to discern the influence of corticothalamic input on the responses of relay neurons in the VB thalamus, they have failed to address the fact that substantial difference exist in the anatomical and functional organization of somatosensory systems between mammals. This application addresses the functional significance of corticothalamic connections within the somatosensory system of adult macaques by examining whether pharmacological interference of corticothalamic neuronal activity from APC alters the response properties of VB relay neurons. In addition, this application directly addresses the mechanisms through which these changes might be taking place by examining whether these changes are associated with alterations in markers for glutamate or GABA or their receptors. It is hoped that this research will provide a greater understanding of the complexity of sensory processing by addressing "top-down" influences over normal activity as well as changes in neuronal responses associated with experience or injury-induced reorganizational plasticity.